For internal combustion engines with injection valves, accuracy and robustness of the injection quantity should be very high under all operating conditions and over the entire service life of the relevant vehicle. To determine an injection valve state, the voltage or charge or current may indicate significant features (e.g., by means of local determination of extreme values). However, in the typical evaluation methods, a large number of influencing factors have to be taken into account, so the methods are very complex since all the relevant interference variables have to be filtered out. Contemporary concepts use feedback signals (e.g., voltage or charge) from a piezo actuator in the injector to identify individual static points of the nozzle needle position during the actual injection process (relying on the piezo-electric effect). However, this information is subject to large interference variable influences because the piezo injector is in use at the same time as both actuator and sensor.